1. Field of the Invention
This invention relates to the field of electronic displays. More specifically the invention relates to drivers for light emitting polymer displays.
2. Background Art
Electronic displays are used in a wide variety of applications today. For instance, digital watches, cellular telephones, computers, handheld electronic diaries (e.g., palm pilot), etc, all use electronic displays. Devices such as printers and copiers use electronic displays to guide the user and provide diagnostic help when necessary. All these devices use different types of display technology which include computer monitors, Liquid Crystal Displays (LCD), and Polymer Light Emitting Diode (PolyLED) displays. Some computer displays, for example, use electronic tubes and others use TFT (thin film transistor) technology. TFT is LCD technology and is commonly used in notebooks and laptop computers. Recently, as the cost of producing TFT displays have dropped, manufacturers have started incorporating them into small portable electronic devices such as telephones and handheld diaries.
PolyLEDs, sometimes called OLED (Organic LED), are thin-film light emitting polymers, sandwiched between a transparent and a metal electrode, a metal backing and a very thin glass or plastic material. The polymer films are arranged in an array of diodes. The polymers emit light when electrons and positive charges are injected from the electrodes and transmitted through the material. PolyLEDs are emissive (like light bulbs) type displays unlike LCDs that are reflective. They are generally arranged as passive matrix displays. Images displayed on a PolyLED display are built up by scanning through the array, sending an intense pulse through each line that is being addressed. The human visual system integrates these pulses into an image with sufficient brightness.
Power Consumption
There are multiple sources of power dissipation in a PolyLED display system (i.e., display and driver). One source is due to the power associated with production of light in the LED, which is the product of the current through the LED and the voltage across the LED. Another source is the resistive losses associated with heating the row and column electrodes in the display. A source of power loss may also be due to precision requirements for current in each pixel thus power may be wasted if precision current sources are required to maintain accuracy between the pixels because some sources may be carrying excess current. Yet another source of power dissipation is due to capacitive losses in charging and discharging the diode capacitances in the display. The capacitance of a PolyLED display is very high because PolyLEDs have thin film polymers sandwiched between two metal plates (i.e., the metal plates are close together). In addition, recent advances in technology have PolyLED displays becoming thinner to fit smaller and lighter devices. Thus, the metal plates are coming closer together therefore increasing the capacitance of the PolyLED displays. Also, in order to reduce power, manufacturers have tended to reduce the current required to produce light to very low levels thus increasing the time used in charging the capacitance of the PolyLEDs. For instance, if the charging time is approximately 50–100 microseconds, and the row time is on the order of 200–300 microseconds, then an unacceptably high percentage of the row time (i.e., pulse width) is used to charge the capacitance. Thus, it is desirable to reduce or eliminate the charging time for each diode in order to preserve a higher percentage of the row time.
An electronic display screen is composed of several pixels. A pixel is the basic unit of programmable color in a computer display. Today's displays typically have thousands of pixels arranged in a matrix of N columns by M rows. As the display gets used over time, some pixels see more current than others. Because not all the pixels are lit all the time, some pixels age faster than others on the same display screen. For instance, when different pictures are displayed on the screen, some pixels will have current for a longer period than others and those pixels that are used more often age faster. The problem with older pixels is that they will not put out as much light as younger pixels when the same voltage is applied across their terminals. Thus, an adaptive method of assuring that each pixel in a display produces approximately the same amount of light is desirable.
Display Drivers
Typically, electronic devices called display drivers provide power to drive the pixels on a display screen. Display drivers are generally built into dedicated Integrated Circuits (ICs). The drivers incorporate all the necessary circuits for proper control of the displays. For the PolyLED display, each column is driven separately by its own circuit which is incorporated into the IC. Thus, for a display screen having a resolution of 102 columns by 65 rows, there are 102 column drive circuits representing one drive circuit for each column and 65 row drive circuits representing one drive circuit for each row.